1. Field of the Invention
The present invention is generally in the field of fabrication of semiconductor devices. More particularly, the invention is in the field of imaging integrated circuits fabricated on a semiconductor wafer.
2. Background Art
Soft defect localization is a well known technique for detecting soft defects, such as timing marginalities, in fabricated semiconductor devices. Soft defect localization typically utilizes a laser to scan regions of a semiconductor device that is concurrently under test. Preferably, the laser is focused on a single transistor at a time to cause localized heating of that transistor as it is being tested. The laser light scattered from the transistor is then collected and analyzed, along with laser light scattered from other device features, to determine the locations of soft defects in the device. However, diffraction effects limit the resolution achievable using this approach. As the dimensions of modern integrated circuits become ever smaller, this traditional technique is no longer capable of isolating individual device features for soft defect analysis.
One conventional solution for performing soft defect analysis of high resolution semiconductor devices employs near field scanning optical microscopy (NSOM). In NSOM, an illuminated optical fiber is positioned very close to a surface being imaged, and is scanned across the surface in a tapping mode. Evanescent light from the tip of the optical fiber is scattered off of the surface, and is detected using a conventional microscope objective lens. Although NSOM has been shown to be effective for high resolution imaging, there are several significant drawbacks to its use in soft defect localization. For example, because the evanescent light from the tip decays very rapidly, only surface or near sub-surface structures can be imaged using the NSOM technique. In addition, scanning the optical fiber tip across the surface is a slow mechanical process, which additionally subjects the tip to erosion and breakage.
Thus, there is a need in the art for a robust high resolution imaging solution suitable for implementation in performing soft defect analysis of integrated circuits and devices.